Thermal mass flowmeters are a common choice for flow metering devices in the commercial and industrial metering markets. A typical sensor element for use in such meters is a resistance temperature detector (RTD), the resistance of which is related to the temperature of the element itself. A typical bridge employs two RTD elements. One of them is referred to as a temperature sensor element and is unheated. An flow sensor RTD element is heated and the effect of mass flow on the heated element provides a measure of the flow velocity of the fluid in a flow tube being monitored. The temperature of the fluid, normally a gas, flowing across the heated RTD is also a factor in the amount of heat dissipated from that RTD.
Two different methods are commonly used to determine the mass flow in a conduit. One is configured to maintain a constant temperature differential between the temperature sensor RTD and the flow sensor RTD. This method measures a bridge signal, such as a voltage or current, required to maintain the flow sensor RTD at a constant temperature above the temperature sensor RTD while heat is removed from the active RTD by way of the physical properties of the flowing fluid. The other method measures a signal difference between the flow sensor RTD and the temperature sensor RTD while the flow sensor RTD is self-heated by a constant current or a constant power heat source. During this measurement, as with the other method, the active RTD loses heat by way of the physical properties of the flowing media.
There are many configurations of dispersion mass flow sensors, and more particularly, of heated RTD type sensors. An early such flow detector is taught in U.S. Pat. No. 3,366,942, “Flow Stoppage Detector,” issued Jan. 30, 1968 to Deane. This patent discloses a reference sensor, a heated or active sensor, and a separate heating element located closely adjacent the heated sensor element. The basic principal of operation of dispersion flowmeters is discussed in this patent. There are many other examples of detectors employing differential temperature sensors, some having three elements as described in the patent mentioned above, and some having two elements, where the active sensor is self-heated. Even a single element differential temperature sensor may be employed. The single element sensor works on a time sharing basis where it acts as a reference sensor part of the time and is then heated to act as the active sensor in relatively rapid succession.
Most of the known differential temperature sensors are configured with the temperature and flow sensors arranged as a Wheatstone Bridge. They are mounted in the fluid conduit and project into the flow path as an insertion flow sensor. The sensor elements are positioned to permit unobstructed flow fluid past both the flow sensor and the temperature sensor in such a way that one does not thermally influence the other. This means that the temperature sensor must indeed be a reference with respect to the fluid being sensed without influence from the heat of the flow sensor or the fluid heated by the heated sensor.
U.S. Pat. No. 4,475,388, “Thermal Flowmeter with Temperature Compensation,” issued Oct. 9, 1984 to Kawai et al. provides a method for measuring the flow rate of a fluid. A signal of the fluid flow rate and a signal of the fluid temperature are produced by using the signals from an electric heater and a first, second, and third temperature-dependent resistors in a signal processing circuit. The produced signals are supplied to a computer circuit to carry out modification, linearization, and multiplication by a conversion constant for linearization which is corrected regarding temperature characteristic.
U.S. Pat. No. 5,544,531, “Flowmeter Having Active Temperature Compensation,” issued Aug. 13, 1996 to Heckman, provides a method and apparatus for measuring fluid flow characterized by compensating for temperature variations in the fluid level pressure transducer. The transducer is operated at a plurality of different combinations of pressure and temperature, and the drive and output voltages of the transducer are measured at each combination. A plurality of temperature coefficients are derived from the measured voltage and entered into a data logger for use in calculating flow.
U.S. Pat. No. 5,237,523, “Flowmeter Fluid Composition and Temperature Correction,” issued Aug. 17, 1993 to Bonne et al., provides a method for correcting the flow measurement of a fluid for changes in the composition and temperature of that fluid in a flowmeter of a hot element type in which an uncorrected flow value signal for the fluid of interest in relation to a hot element sensor output is corrected by applying a first correction factor to the output based on certain unique physical parameters of the fluid of interest which nominally include thermal conductivity, specific heat, and temperature, obtaining an uncorrected flow measurement value from the corrected output and obtaining the corrected flow measurement by applying a second correction factor to the uncorrected flow measurement value based on the unique physical parameters.
In this art, there is a concern in the previously proposed arrangements for determining a flow rate of the fluid to a high degree of accuracy over a wide temperature range and flow rate. While the prior techniques improve the accuracy of the flow rate determination over a somewhat narrow temperature range, a flowmeter having high accuracy over a wide temperature and flow rate range would be highly desirable.